Thermal paper comprises a base sheet and a thermosensitive coating with color forming chemicals therein such that when heat is applied to the paper by a thermal print head, the color forming chemicals react to develop color. The application of heat is controlled to form the desired print or image.
The most common thermosensitive coatings employ a dye-developing type color forming system. There are three main color producing components in a dye developing-type thermosensitive coating and they are: a colorless dye (color former), a bisphenol or acidic material (color developer) and a sensitizer. Images are formed in the thermosensitive coating by the application of heat to melt and interact these three color producing materials.
Certain chemical factors can adversely affect and degrade the performance of the thermosensitive coating on thermal paper. These chemical factors include certain organic solvents (ketones), plasticizers (polyethylene glycol type) amines (ammonia) and oils (soy oil).
Simply handling thermal papers with certain color forming compounds can result in premature coloration unless a barrier layer or protective top coating is coated over the thermosensitive coating, (See U.S. Pat. Nos. 4,370,370; 4,388,362; 4,424,245; 4,444,819; 4,507,669; 4,551,738 and 4,604,635) or the color forming compounds are encapsulated in microcapsules which release their contents when exposed to heat, (See U.S. Pat. Nos. 4,682,194; 4,722,921; 4,740,495; 4,742,043; 4,783,493; and 4,942,150).
There are many security inks available which serve to thwart the duplication of printed commercial documents by providing latent images or images that change color when exposed to a light source other than ambient light. Fluorescent inks are one example. Conventional fluorescent inks typically contain a fluorescent compound which responds to infrared or ultraviolet light. An example of a printing ink which fluoresces under ultraviolet radiation is described in U.S. Pat. No. 4,153,593. The dyes described in this reference include fluorescein dyes, eosine dyes and Rhodamine dyes. Other ink formulations are disclosed in U.S. Pat. No. 4,328,332, issued to Hayes et al.; U.S. Pat. No. 4,150,997, issued to Hayes; U.S. Pat. Nos. 4,452,843 and 4,598,205 issued to Kaule et al., and U.S. Pat. No. 5,503,904, issued to Yoshinaga et al.
The use of conventional fluorescent inks on thermal papers has resulted in pre-reaction of the reactive components within the thermosensitive coating of the thermal paper or disruption of the color forming reaction when heated. The conventional protective top coatings and microcapsules mentioned above have not been effective in preventing premature coloration of the thermosensitive coating or disruption of the color forming reaction when exposed to conventional fluorescent security inks. As a result, special measures have been taken to incorporate security features in thermal papers. U.S. Pat. Nos. 5,883,043; 6,245,711 and 6,562,755 disclose thermal papers with security inks printed on the side opposite the thermosensitive coating and in U.S. Pat. Nos. 6,060,426; 6,106,910; 6,165,937; and 6,613,403, special near infrared fluorescent (NIRF) compounds are employed as a security feature or sense mark for thermal papers. The NIRF compounds are fluorescent compounds which respond to wavelengths in the range of 650 nm to 2500 nm and are very sensitive and unstable. Amounts as low as 0.1 ppm solids can be detected, permitting the NIRF compounds to be incorporated in the base sheet, a base coating, the thermosensitive coating or an optional top coating, with minimal interaction with the color forming compounds in the thermosensitive layer.
It is desirable to provide a thermal paper with a fluorescent security mark printed over the thermosensitive coating using a stable fluorescent compound.